Through those eight years, I think that's when those other 500 or so criminals were detained/arrested/whatever. And Selina Kyle's criminal past was a huge factor on why she was sent to Blackgate Prison as well. If she worked with someone like Daggett then she could've worked with mobs beforehand as well.

Right, she had a huge record. My point was just that the movie shows that the Dent Act is more sweeping and powerful than a legislation that simply denies people who were already in jail parole. It allowed them to lock up a woman in an all-male facility, when that previously wouldn't have been possible in Gotham. Denial of parole and gender non-segregation are two aspects of the Dent Act we're explicitly told about, but common sense dictates that there has to be more to it in order for them to have wiped out organized crime as we're shown in the film. This is where a splash of imagination can do wonders.